Mineralogical Magazine, August 2004, Vol. 68(4), pp. 541–559

The minor intrusions of , NW : early development of magmatism along the Caledonian Front

1, 2,3 4 K. M. GOODENOUGH *, B. N. YOUNG AND I. PARSONS 1 British Geological Survey, West Mains Road, EH9 3LA, UK 2 Department of and Mineralogy, University of Aberdeen, Marischal College, Broad Street, Aberdeen AB24 3UE, UK 3 Baker Hughes Inteq, Barclayhill Place, Portlethen, Aberdeen AB12 4PF, UK 4 Grant Institute of Earth Science, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK

ABSTRACT The Assynt Culmination of the , in the northwest , contains a variety of Caledonian alkaline and calc-alkaline intrusions that are mostly of Silurian age. These include a significant but little-studied suite of dykes and sills, the Minor Intrusion Suite. We describe the structural relationships of these minor intrusions and suggest a classification into seven swarms. The majority of the minor intrusions can be shown to pre-date movement in the Moine Thrust Belt, but some appear to have been intruded duringthe period of thrusting.A complex history of magmatism is thus recorded within this part of the Moine Thrust Belt. New geochemical data provide evidence of a subduction-related component in the mantle source of the minor intrusions.

KEYWORDS: Assynt, Caledonian, minor intrusion, Moine Thrust, Scotland.

Introduction north of Assynt, to the Achall valley near , but they are most abundant in the Assynt area. The WITHIN the Assynt Culmination of the Moine minor intrusions constitute a significant part of the Thrust Belt of NW Scotland (Fig. 1) lie a variety of total volume of igneous rock in the Assynt area, Caledonian alkaline and calc-alkaline intrusions but have been little studied. The first detailed study that have been of interest to geologists since the was that of Sabine (1953) who divided the minor end of the 19th century (e.g. Bonney, 1883; Heddle, intrusions into six classes and studied their 1881; Horne and Teall, 1892; Teall, 1900; Teall, relationships to the regional structure. Subsequent 1907; Shand, 1910; Phemister, 1926). The main publications on the region (e.g. Parsons, 1979) syenite intrusions of Assynt, the Loch Borralan and have generally accepted the detailed work of Loch Ailsh plutons, have been well studied (e.g. Sabine; a review of the existingliterature is Parsons, 1965; Woolley, 1970; van Breemen et al., provided by Parsons (1999). The only recent 1979; Thompson and Fowler, 1986, Halliday et al., investigation of these intrusions was in a hitherto 1987; Thirlwall and Burnard, 1990; Parsons, unpublished thesis (Young, 1989). We present new 1999). Associated with them is a less well field, petrographical and geochemical data, on the understood suite of minor intrusions, formally basis of which we have revised the original known as the Northwest Highlands Minor classification scheme for the minor intrusions. Intrusion Suite, which includes sills and dykes The relationships between the different swarms that range in composition from lamprophyres to and their possible sources are discussed below. peralkaline rhyolites and nepheline-syenites. The intrusions of this suite extend from Loch More, Regional geology * E-mail: [email protected] The Assynt Culmination is the largest and most DOI: 10.1180/0026461046840207 geologically spectacular window through the

# 2004 The Mineralogical Society K. M. GOODENOUGH ETAL.

FIG. 1. Simplified map of the northwest of Scotland, showing the major geological structures.

Moine Thrust Belt (Fig. 2) (Johnson and Parsons, have been disputed (Peach et al., 1907; Bailey, 1979; Elliott and Johnson, 1980; Coward, 1983, 1935; Sabine, 1953; Krabbendam and Leslie, 1985). West of the Assynt Culmination lies the 2004). Movingwest from the Moine Thrust, these undisturbed Foreland (Park et al., 2002), where are: the Ben More Thrust, with a sheet of the clastic sedimentary rocks of the Proterozoic Lewisian gneisses, Torridonian sandstones and Torridonian Supergroup overlie Archaean Cambrian quartzites in its hangingwall; the Lewisian gneisses along an irregular unconfor- Glencoul Thrust, carryingLewisian gneissesand mity. The Torridonian rocks are in turn overlain Cambrian quartzites; and the Sole Thrust, by a Cambro-Ordovician sequence, consistingof carryingduplexes of the Cambro-Ordovician thick quartzite units that pass up into fissile succession. The amount of movement on the mudstones (the Fucoid Beds) and gritty sand- Sole Thrust has been a cause of some debate stones, with the dolomitic limestones of the (Coward, 1985; Halliday et al., 1987) and this has Group at the top of the sequence. To not yet been resolved. the east, the Assynt Culmination is bounded by The major igneous intrusions in the Assynt the Moine Thrust, east of which lie Proterozoic region are the alkaline Loch Borralan and Loch metasedimentary rocks of the . Ailsh syenite plutons. The Loch Borralan Pluton Within the Assynt Culmination, it is generally is in part silica undersaturated and highly potassic agreed that three major thrusts can be recognized (Woolley, 1970) whereas all units at Loch Ailsh (Fig. 2), although the exact lines of the thrusts are silica saturated and relatively sodic (Parsons,

542 EARLY MAGMATISM ALONG THE CALEDONIAN FRONT, SCOTLAND

FIG. 2. Sketch map of the Assynt Culmination, showingthe approximate distribution of the main groupsof intrusive rocks. Thrust patterns from Peach et al. (1907), modified after Coward (1983) and Krabbendam and Leslie (2004). MT À Moine Thrust; BMT À Ben More Thrust; GT À Glencoul Thrust; ST À Sole Thrust. Locations of minor intrusions from Sabine (1953), modified after Young(1989) and recent BGS mapping.Note that this is not a comprehensive map of all the minor intrusion localities.

1965). Both plutons are Silurian in age: the Loch al., 1987). The Loch Borralan Pluton is Borralan Pluton has a U-Pb date of 430Ô4 Ma considered to post-date movements on thrusts (van Breemen et al., 1979); and the Loch Ailsh within the Moine Thrust Belt (Parsons and Pluton has been dated at 439Ô4 Ma (Halliday et McKirdy, 1983), whereas the Loch Ailsh Pluton

543 K. M. GOODENOUGH ETAL.

TABLE 1. Summary of the modern rock names for the Assynt minor intrusions.

Name given by Modern rock name Modern swarm Structural Sabine (1953) name relationship

Canisp Porphyry Porphyritic quartz- Porphyry Swarm Pre-thrustingin microsyenite Moine Thrust Belt Ledmorite Melanite nepheline- Ledmorite Swarm Uncertain microsyenite Grorudite Peralkaline rhyolite Peralkaline Rhyolite Swarm Pre-thrustingin Moine Thrust Belt Hornblende- Hornblende microdiorite Hornblende Microdiorite Swarm Pre-thrustingin porphyrite Moine Thrust Belt Nordmarkite Quartz-microsyenite Nordmarkite Swarm Predates ductile movement on Moine Thrust Vogesite Vogesite Vogesite Swarm Pre-thrusting in Moine Thrust Belt Not recognized Porphyritic trachyte Porphyritic Trachyte Swarm Syn- to post-thrusting in Moine Thrust Belt

has generally been considered to pre-date area for the Canisp Porphyry is on Beinn Garbh, to thrusting(Halliday et al., 1987). the south of , where it forms a series of sills of which at least one is nearly 50 m thick. Structural relationships of the Assynt minor intrusions Vogesites Sabine (1953) divided the minor intrusions of The vogesites are green-grey-weathering Assynt into six main classes: ‘grorudite’; ‘Canisp medium- to coarse-grained hornblende-phyric Porphyry’; ‘hornblende porphyrite’; ‘nordmarkite lamprophyres that form sills up to many tens of porphyry’; ‘vogesite’ and ‘ledmorite’. We have metres thick. They occur throughout the Assynt updated this classification on the basis of new Culmination, but are most common in the field, petrographical and geochemical data (see Cambro-Ordovician dolomitic limestones of the Table 1 and also Parsons, 1999). New rock names Sole Thrust Sheet (Fig. 2). They are not seen are based on the BGS Rock Classification Scheme within the Foreland or east of the Moine Thrust. Volume 1: Igneous Rocks (Gillespie and Styles, The best evidence that these intrusions pre-date 1999), which is closely based on the IUGS thrustingcan be seen at the roadside on the north scheme (Le Maitre et al., 1989) and is now used shore of Loch Assynt (NC 240239), where an in all BGS maps and publications to ensure offshoot from a vogesite sill in dolomitic lime- consistency. Detailed field mappinghas givenus a stone is clearly imbricated. Some vogesite sills in clear understandingof the relationship of the the Loch Ailsh area have been hornfelsed (Young, different swarms to each other, and to the regional 1989) by the intrusion of the Loch Ailsh Pluton structures. (439 Ma; Halliday et al., 1987), which they must therefore pre-date. The vogesites are thus considered to be amongthe earliest intrusions in Canisp Porphyry the Moine Thrust Zone in Assynt. The Canisp Porphyry is a distinctive red feldspar- phyric quartz-microsyenite (Sabine, 1953). It has Hornblende microdiorites been recognized only in the Foreland (Fig. 2), close to the Sole Thrust but never east of it, and is Sabine’s ‘hornblende porphyrites’ have been generally considered to pre-date movement in the renamed hornblende microdiorites (Parsons, Moine Thrust Belt (e.g. Parsons, 1999). The type 1999). These are medium-grained rocks, which

544 EARLY MAGMATISM ALONG THE CALEDONIAN FRONT, SCOTLAND

can be distinguished from the vogesites in hand of porphyritic trachyte cut across foliations that specimen by the presence of both feldspar and are related to movements on the Ben More Thrust, hornblende phenocrysts. They are remarkably indicatingthat the porphyritic trachytes are later consistent in appearance across the area, intrusions. However, at other localities the commonly formingsills that are mostly only a porphyritic trachytes may be cut off by late few metres thick but in places up to tens of movements alongthrusts, althoughthe critical metres. The hornblende microdiorites are abun- relationships are almost always obscured due to a dant throughout the Assynt Culmination and, as lack of exposure. observed by Sabine (1953), they clearly pre-date thrusting, being folded and deformed at many Nordmarkite Swarm locations. This suite of intrusions was termed ‘‘nordmarkitic rocks’’ by Sabine (1953) followingterminology Peralkaline rhyolites used by Phemister (1926). Parsons (1999) noted The majority of intrusions belonging to the that these sills are in fact porphyritic quartz- ‘grorudite’ class of Sabine (1953) are now microsyenites, but the name ‘Nordmarkite classified as peralkaline rhyolites. These intru- Swarm’ is retained here in order to distinguish sions are typically brick-red or green on this group of about eight tectonically deformed weathered surfaces. They form sills and dykes, sills from other, undeformed quartz-microsyenites generally only 1 m thick, although a few thicker in Assynt. intrusions have been recognized. Sabine (1953) Sabine (1953) noted that the quartz-micro- considered that the peralkaline rhyolites were syenites of the Nordmarkite Swarm crop out restricted to the Glencoul and Ben More thrust along, close to, and on both sides of, the Moine sheets, includingthe klippen of the Ben More Thrust plane. This restricted extent has been taken Thrust (Fig. 1). However, we have now recog- to suggest that most movement on the Moine nized peralkaline rhyolites in several locations Thrust had occurred before these sills were beneath the Ben More Thrust, includingexamples intruded (Parsons, 1999), and that the sills were just above the Sole Thrust at Knockan and at Cam intruded alongthe line of the thrust. However, we Loch, and within imbricated limestones at Cnoc have recognized some sills of the Nordmarkite an Uamh in Glen Traligill. Many of the peralka- Swarm several hundred metres east of the Moine line rhyolite intrusions can be clearly shown to Thrust. Mylonitic fabrics are seen within the pre-date thrusting, since they are deformed by margins of the intrusions where they are emplaced thrust-related folding (e.g. on Cnoc an Droighinn; into mylonitic Moine rocks, indicatingthat they Coward and Potts, 1985). Peralkaline rhyolite sills must have been intruded prior to some, if not all, cut sills of hornblende microdiorite on the slopes of the ductile movements on the Moine Thrust. of Cnoc an Droighinn above , confirmingthat the peralkaline rhyolites are the Ledmorites youngest of the pre-thrusting minor intrusions. A sill of peralkaline rhyolite is also seen cutting The ledmorites (melanite augite nepheline-micro- syenites of the Loch Ailsh Pluton in Glen Oykel syenites) described by Sabine (1952, 1953) form a (Parsons, 1999). few brick-red, fine-grained dykes cutting rocks of the Foreland on the west coast, at and . They are believed to be related to Porphyritic trachytes the Loch Borralan Pluton (Parsons, 1999). Little A few intrusions described by Sabine (1953) as new work has been carried out on these intrusions grorudites have now been recognized as a distinct and so they are not included in the following group of porphyritic trachytes. These also form sections. brick-red weatheringsills and dykes with small feldspar phenocrysts, but are chemically and petrographically distinct from the peralkaline Petrography rhyolites. The porphyritic trachytes generally The petrography of the minor intrusions was seem to be concentrated close to the Ben More described briefly by Sabine (1953). The following Thrust. At two localities, one on the slopes of section summarizes his work, together with our and the other below Sgonnan Mor, dykes new observations.

545 K. M. GOODENOUGH ETAL.

Canisp Porphyry groundmass (Sabine, 1953). The feldspar pheno- crysts include both microperthitic alkali feldspars The Canisp Porphyry is characterized by large (up up to 3 mm, and larger sodic plagioclase crystals. to 15 mm) euhedral feldspar phenocrysts in a fine- They are typically sericitized, and aggregates of grained groundmass that is composed of alkali several crystals are common. Many of the crystals feldspar crystals <0.1 mm across, micropoikiliti- show oscillatory or complex sector zoning. cally enclosed by quartz plates up to 0.5 mm in Hornblende phenocrysts range in length from size (Sabine, 1953). Phenocrysts are predominantly 0.5 to 3 mm, and some are zoned. Plates of sodic plagioclase with rare microperthitic and altered biotite are also present in some samples. cryptoperthitic alkali feldspar phenocrysts; some examples of both types have mantles of albite, Peralkaline rhyolites turbid in places. Some larger phenocrysts are composed of aggregates of several feldspar The peralkaline rhyolites commonly have a fine- crystals. Pale green pyroxene phenocrysts are are grained groundmass, with alkali feldspar pheno- up to 3 mm long, euhedral, with diopsidic cores crysts up to 5 mm across that are typically turbid mantled by aegirine-augite rims. Tabular biotite and have been largely recrystallized to coarse patch phenocrysts, up to 3 mm long, are commonly perthite. Fresher examples of these phenocrysts altered to chlorite. Small euhedral phenocrysts of contain areas of lamellar microperthites or are titanite and apatite are seen in some samples. featureless and therefore probably cryptoperthitic, and some are crowded with tiny aligned needles, probably of aegirine. Many phenocrysts appear to Vogesites be glomerocrysts of irregular alkali feldspar grains The vogesites can be divided into melanocratic and and occasional plagioclase, which appears to have leucocratic varieties, on the basis of the abundance been largely replaced. Euhedral phenocrysts of of mafic minerals and the presence or absence of aegirine-augite and titanite are found in some pyroxene. A typical melanocratic vogesite consists samples. The groundmass consists of fine-grained of ~50% mafic minerals in a felsic groundmass, quartz and feldspar crystals, together with stubby with phenocrysts of both diopsidic pyroxene and needles of aegirine that commonly define a hornblende occurringin all samples. The ground- magmatic flow direction. To the south of Glen mass is composed of an interlockingmass of Traligill (Fig. 2), the peralkaline rhyolites are irregular grains of alkali feldspar, up to 3 mm petrographically variable. Some coarse-grained across, and commonly turbid. The hornblende examples from Sgonnan Mor have feldspar phenocrysts vary in size from 1 to 7 mm in phenocrysts up to 7 mm across. Other peralkaline different samples, and are usually subhedral. The rhyolites from the Sgonnan Mor-Loch Ailsh area pyroxene phenocrysts show variable degrees of contain quartz and/or sodic amphibole phenocrysts. alteration; in some samples they form colourless, Microprobe analyses show that the range of euhedral crystals up to 1 mm long, but in many pyroxene compositions in the peralkaline rhyolites other samples the pyroxenes are represented by is very large, from Ac10Di75Hd15 to nearly pure clusters of euhedral crystals that have been aegirine (Young, 1989). extensively altered to chlorite-rich aggregates. Calcite is present in all the pyroxene-bearing Porphyritic trachytes vogesites, and commonly appears to be replacing primary minerals or formingvein fills. The porphyritic trachytes contain phenocrysts of The leucocratic vogesites contain sparse sodic plagioclase up to 7 mm long, in a fine- phenocrysts of hornblende up to 4 mm long, in grained groundmass that consists almost entirely a groundmass composed mainly of plates of alkali of aligned laths of alkali feldspar. Unaltered mafic feldspar. Calcite is absent, as is pyroxene; rare silicate phenocrysts have not been recognized, but plates of biotite are typically altered to chlorite. irregular aggregates of opaque oxides may represent pseudomorphs of original mafic phases. Hornblende microdiorites Nordmarkite Swarm The hornblende microdiorites are strongly porphyritic, with phenocrysts of feldspar and The rocks of the Nordmarkite Swarm are hornblende occurringin a fine-grainedfeldspathic medium- to coarse-grained quartz-microsyenites,

546 EARLY MAGMATISM ALONG THE CALEDONIAN FRONT, SCOTLAND

with phenocrysts of turbid alkali feldspar (largely suite of appinitic minor intrusions within the perthitic, but also includingdiscrete sub-crystals Moine Supergroup, some 50 km to the east of of microcline and albite) in a medium-grained Assynt. Total alkalis are from 4À8 wt.% in the groundmass. The phenocrysts are in places up to vogesites and hornblende microdiorites, rising to 5 mm long, but are more commonly 1À2 mm, 8À10 wt.% in the peralkaline rhyolites and higher and in places show signs of strain such as kinked in some of the porphyritic trachytes (Fig. 3). In twins. Small (up to 1 mm) phenocrysts of general, the Assynt minor intrusions show a clinopyroxene (augite) are also seen in some tendency for alkali enrichment with increasing samples. The groundmass typically consists of silica content, with a correspondingdepletion in quartz and feldspar with epidote, biotite, chlorite Fe2O3, MnO, MgO, CaO and TiO2. Although the and calcite, plus accessory zircon and sulphides. Assynt minor intrusions have generally been A distinctive feature of many of the samples of considered to be related to the NW Highlands the Nordmarkite Swarm, particularly those from alkaline plutonic suite, the vogesites and horn- the margins of sills within mylonites just above blende microdiorites are more strictly calc-alka- the Moine Thrust, is that the groundmass shows a line (Fig. 4). The peralkalinity index (molar strongpenetrative tectonic fabric. In comparison (Na2O+K2O)/Al2O3)) of the peralkaline rhyolites with undeformed parts of the sills, the crystals averages about 1, i.e. some of them are not truly within the groundmass in deformed samples have peralkaline, although they do typically have undergone a pronounced grain-size reduction, to normative acmite. about a half or a third of their original size. In The trace-element characteristics of the minor many deformed samples the fabric wraps the intrusions are illustrated on multi-element plots, alkali feldspar phenocrysts in an asymmetrical normalized to primitive mantle (Fig. 5). The manner, and strain-shadows filled by quartz occur vogesites, hornblende microdiorites, Canisp adjacent to some alkali feldspar phenocrysts. This Porphyry sills and intrusions of the Nordmarkite shear fabric is in places cut across by later calcite Swarm all have very similar patterns, with a clear veins. In general, the fabric in the sheared quartz- Nb trough and relatively high Ba and Sr contents. microsyenites indicates that these rocks have The porphyritic trachytes and peralkaline rhyo- undergone significant amounts of ductile defor- lites show a wider range in their trace element mation together with the surrounding mylonitized patterns. The majority of these more evolved Moine country rocks. intrusions have substantially lower Ba and Sr contents than the more mafic rocks, and do not have such a pronounced trough at Nb. Zr contents Geochemistry are generally higher, with the peralkaline Bulk-rock samples of most of the different types rhyolites havingthe highestZr values. of minor intrusions, with the exception of the Nordmarkite Swarm, were analysed by X-ray fluorescence (XRF) for major and trace elements Discussion at Aberdeen University by Young(1989). Previous authors have considered the minor Selected further samples of peralkaline rhyolites, intrusions of Assynt as a related, cogenetic suite porphyritic trachytes and quartz-microsyenites of (Sabine, 1953; Parsons, 1999). However, there are the Nordmarkite Swarm have been analysed as significant variations in structural relationships part of the current BGS Moine Thrust Project. and in geochemistry between the different These analyses were carried out by XRF, using intrusion swarms, and it is therefore necessary the PW2400 spectrometer and standard proce- to consider the possibility that more than one dures employed by the UKAS-accredited analy- magma source may have been present. The nature tical labs at BGS Keyworth. Major elements were of the magma source, or sources, and the analysed on fused beads and trace elements on derivation of the different magma types, are pressed powder pellets. discussed below. The Assynt minor intrusions range from The most primitive of the Assynt Caledonian vogesites with SiO2 contents of 52À60 wt.% to magmas were those that formed the mafic peralkaline rhyolites that typically have SiO2 vogesites, and these are the most likely parental contents in excess of 75 wt.% (Fig. 3). This is a magmas for the rest of the Assynt minor similar range to that described by Fowler and intrusions. These magmas were clearly mantle Henney (1996) for the Ach’ Uaine Hybrids, a derived, since they have high contents of MgO

547 K. M. GOODENOUGH ETAL.

FIG. 3. Total alkalis vs. silica diagram for the Assynt minor intrusions. Data for minor intrusion swarms from Young (1989) and Tables 2 and 3. Data for Loch Borralan syenites from Thirlwall and Burnard (1990). Classification fields from Gillespie and Styles (1999) after Le Bas et al. (1986).

FIG. 4. AFM diagram for the Assynt minor intrusions. Data for minor intrusion swarms from Young (1989) and Tables 2 and 3. Tholeiitic/calc-alkaline division from Irvine and Baragar (1971).

548 EARLY MAGMATISM ALONG THE CALEDONIAN FRONT, SCOTLAND

FIG. 5. Multi-element plots, normalized to primitive mantle, for selected samples of the Assynt minor intrusions. Data from Tables 2 and 3 and from Young(1989). Other NW Highlandintrusions are shown for comparison; data from Thompson and Fowler (1986). Normalizingvalues from McDonoughand Sun (1995).

(up to 10%) and Ni (up to 140 ppm, although Major-element modellingexperiments, based more typically 50À60). on the method of Stormer and Nicholls (1978),

549 K. M. GOODENOUGH ETAL. were carried out by Young(1989), and the data are available in that thesis or from the authors. The models can only be taken as approximations, since all the minor intrusions contain phenocrysts and thus rock samples do not necessarily represent true liquid compositions. Young showed that the hornblende microdiorites could have been generated from the same magma that formed the melanocratic vogesites, through simple crystal fractionation of hornblende, plus smaller amounts of diopsidic pyroxene, forsteritic olivine and Fe-Ti oxides. All of these minerals occur as phenocrysts in the vogesites, and the chemical compositions of the main fractionating phases are illustrated together with rock composi- tions in Fig. 6. The Canisp Porphyry sills are chemically similar to the hornblende microdior- ites and could have also formed through fractionation of a vogesite-type parental magma, although there are no vogesite intrusions in the unthrusted Foreland. Thompson and Fowler (1986) suggested that the peralkaline rhyolites of Assynt were poten- tially formed through fractionation of alkali feldspar, clinopyroxene, Fe-Ti oxide, apatite, sphene and allanite from the magmas that formed the nearby syenite plutons. However, the peralkaline rhyolites occur throughout Assynt and are not spatially associated with the major syenite bodies. Furthermore, Thompson and Fowler (1986) studied La/Nb ratios, which do not vary significantly during fractional crystallization, and showed that the majority of syenites and other Caledonian igneous rocks in the Northern Highlands have very high La/Nb ratios (>5). La data are not available for all the types of minor intrusion, but La/Nb for the peralkaline rhyolites and porphyritic trachytes ranges from 0.3 to 3.2. Derivation of the peralkaline rhyolites through fractionation of a syenite magma thus seems to be unlikely. We consider it more likely that the Assynt peralkaline rhyolites formed by extensive crystal fractionation of plagioclase and hornblende from the vogesite and hornblende microdiorite magmas (Fig. 6). The Ba and Sr depletion in the peralka- 2. Selected XRF data from Young(1989). Full sample lists are available in Young(1989) or from the authors.

line rhyolites relative to the other minor intrusion 0.75 0.84 0.840.37 0.8 0.43 0.7 0.4 0.85 0.48 1 0.29 1.4 0.3 1.02 0.57 0.94 0.76 1.11 0.71 0.93 0.27 0.57 0.47 ABLE 54.0710.77 54.64 13.02 54.2 12.3 57.35 16.09 59.63 14.98 56.1 13.72 55.01 14.36 53.58 16.63 53.61 14.6 54.61 14.24 54.82 15.8 57.13 15.91 T suites could be explained by the fractionation of vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite plagioclase from an evolving hornblende micro- diorite magma, and the large range of pyroxene compositions (Young, 1989) is consistent with this suggestion. However, evolution through tot 8.46 7.65 7.94 6.09 6.86 7.99 7.89 8.73 7.54 9.22 8.61 7.42 3 3 5 2 2 O 2.23 2.45 2.6 4.39 3.18 3.5 2.74 3 2.29 3.03 2.9 2.03 O O 2 fractional crystallization alone may not explain O 1.96 2.6 3.35 2.99 3.21 2.35 2.05 3.79 3.59 1.71 2.09 3.17 2 2 O 2 2 Al LOITotal 2.19 100.04 98.38 3.8 99.56 3.55 99.77 3.38 99.89 1.92 98.89 100.14 1.57 99.78 1.89 99.73 1.53 99.13 3.43 100.53 102.05 1.9 2.14 4.42 TiO Fe P MnOMgOCaONa 0.14 10.65 8.45 0.11 6.99 5.85 0.11 8.3 5.97 0.09 3.21 4.9 0.11 4.51 4.5 0.15 6.35 0.17 6.01 8.3 0.11 6.16 4.64 0.11 5.61 7.43 0.14 5.4 6.87 0.13 6.2 6.58 0.1 5.78 5.84 4.63 Major elements, wt.% SiO K the distinctly higher Nb and Zr contents in the Sample typeSample number Mafic AS28 Mafic AS29 Mafic AS32 Mafic AS35 Mafic AS37 Mafic AS39 Mafic 2V1 Mafic Mafic V3 Mafic V4 Mafic V5 Mafic V6 BS18

550 Trace elements, ppm Y 252421161511243323171714 Nb13131366371415435 Sr 1411 1427 864 1328 1235 1367 1001 871 1816 820 2891 1046 Rb 39 42 67 37 35 38 26 57 53 25 17 47 Zr 126 128 160 93 87 42 119 195 99 101 0 110 AL AMTS LN H AEOINFOT SCOTLAND FRONT, CALEDONIAN THE ALONG MAGMATISM EARLY Ni 69 70 37 21 34 53 151 14 97 47 45 43 Ba 594 1085 697 892 1178 1129 762 979 1043 790 725 1762

Sample type Leuco Leuco Leuco Leuco Leuco Leuco Leuco Leuco Leuco Leuco Leuco Leuco vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite vogesite Sample number AS6 AS7 AS40 BS2 BS5 BS6 BS7 BS8 V5 V8 V9 V10

Major elements, wt.% SiO2 57.06 58.03 59.62 56.82 53.3 57.15 56.4 56.65 54.94 57.96 57.5 57.47 TiO2 0.65 0.67 0.57 0.77 1.09 0.86 0.86 0.83 0.92 0.87 0.86 1.02 Al2O3 13.89 14.02 14.45 14.63 15.16 16.35 16.03 16.18 16.03 16.01 15.97 15.76 551 Fe2O3 tot 6.59 6.87 6.52 8.15 8.67 7.26 7.17 7.02 7.23 7.14 6.92 7.02 MnO 0.1 0.11 0.11 0.13 0.11 0.13 0.11 0.12 0.08 0.09 0.13 0.12 MgO 5.31 4.95 6.11 6.51 7.52 4.18 4.12 3.96 5.36 4.91 5.25 5.39 CaO 5.1 5.37 3.93 4.76 5.29 3.97 4.65 5.09 4.17 3.56 3.25 4.76 Na2O 2.06 2.87 3.26 2.81 2.08 3.42 2.71 2.85 1.97 3.29 2.4 2.73 K2O 1.98 2.14 2.21 2.55 2.79 2.4 2.77 2.8 3.06 2.56 2.95 2.12 P2O5 0.2 0.21 0.33 0.3 0.36 0.43 0.36 0.45 0.54 0.5 0.47 0.56 LOI 6.36 3.87 2.2 2.3 3.1 2.9 3.55 3.31 5.66 3.88 4.54 3.48 Total 99.3 99.11 99.31 99.73 99.47 99.05 98.73 99.26 99.96 100.77 100.24 100.43

Trace elements, ppm Y 141616161917141318141516 Nb724254454545 Sr 711 1102 1291 1945 1020 2621 2857 3394 929 690 848 951 Rb 36 30 30 36 31 33 36 46 79 32 52 32 Zr 117 92 114 58 124 10 0 0 121 129 115 104 Ni 26 27 139 80 96 22 19 19 34 33 31 32 Ba 540 921 905 818 806 1118 946 1048 629 882 916 811 TABLE 2(contd.)

Sample type Hornblende Hornblende Hornblende Hornblende Hornblende Hornblende Hornblende Hornblende Hornblende Hornblende Hornblende Hornblende microdiorite microdiorite microdiorite microdiorite microdiorite microdiorite microdiorite microdiorite microdiorite microdiorite microdiorite microdiorite Sample number P1A P6 P9 P10 P11 P12 AS36 AS38 AS39 AS42 AS43 AS44

Major elements wt.% SiO2 60.83 58.82 61.8 61.91 61.07 61.27 65.19 64.54 65.51 65.19 64.59 64.34 TiO2 0.83 0.85 0.62 0.56 0.65 0.6 0.44 0.42 0.43 0.51 0.44 0.44 Al2O3 17.24 16.41 15.6 15.6 15.37 15.17 14.33 14.25 14.36 16.18 14.2 14.05 Fe2O3 tot 6.34 6.51 6.21 5.58 6.51 6.02 4.54 4.18 4.38 5.01 4.42 4.45 MnO 0.11 0.08 0.1 0.11 0.11 0.1 0.07 0.07 0.09 0.1 0.08 0.09 MgO 3.68 4.23 3.38 2.72 4.13 3.33 3.71 3.83 3.78 2.24 3.62 3.63 CaO 3.95 3.48 3.09 2.98 3.4 3.8 3.37 3.9 3.66 3.76 3.37 3.35 Na2O 2.6 2.62 3.81 3.24 3.89 3.95 4.65 3.29 4.37 3.91 4.87 4.28

K2O 2.01 2.93 2.44 2.57 1.61 1.71 1.98 2.3 2.02 2.21 2.08 2.24 GOODENOUGH M. K. P2O5 0.51 0.6 0.19 0.19 0.2 0.17 0.12 0.14 0.14 0.18 0.13 0.14 LOI 1.87 4.13 2.1 3.03 2.1 3.18 1.16 3.43 1.15 1.4 1.09 1.9 Total 99.97 100.66 99.34 98.49 99.04 99.3 99.56 100.35 99.89 100.69 98.89 98.91

552 Trace elements, ppm Y 17121513161415912161111 Nb653653634533 Sr 1148 1070 1093 987 876 1014 1010 1234 1090 1340 1193 1313 Rb 27 57 31 40 16 22 25 43 47 39 38 34 ETAL. Zr 104 100 107 108 122 105 19 42 52 98 50 49 Ni 21 31 22 18 27 21 58 57 48 18 53 55 Ba 718 1085 1091 1094 946 1196 1206 1287 811 1071 1114 1175

Sample type Hornblende Hornblende Hornblende Hornblende Hornblende Hornblende Hornblende Peralkaline Peralkaline Peralkaline Peralkaline Peralkaline microdiorite microdiorite microdiorite microdiorite microdiorite microdiorite microdiorite rhyolite rhyolite rhyolite rhyolite rhyolite Sample number AS45 AS51 AS56 BS9 BS10 BS22 BS24 BS11 GR06 GR06R GRO7 GRO8

Major elements, wt.% SiO2 65.01 62.22 62.34 65.35 59.97 59.17 65.47 77.22 73.92 76.62 71.61 77.57 TiO2 0.46 0.58 0.57 0.45 0.83 0.81 0.45 0.05 0.1 0.06 0.11 0.08 Al2O3 14.33 14.85 15.79 16.32 17.29 17.09 16.66 13.18 12.95 12.01 15.21 11.71 Fe2O3 tot 4.57 5.92 5.79 3.54 6.82 6.6 3.57 0.62 1.37 1.5 1.23 1.41 MnO 0.09 0.1 0.09 0.09 0.1 0.12 0.08 0 0.03 0.02 0.03 0.01 MgO 3.97 3.99 2.44 1.73 3.12 3.37 1.71 00000 CaO 3.87 4.05 4.31 2.5 4.51 5.04 3.04 0 0.11 0.26 0.3 0 Na2O 4.48 3.51 3.64 4.66 3.71 3.54 4.36 5.32 5.77 5.35 6.76 5.48 K2O 2.2 2.94 2.33 2.62 2.1 1.87 2.42 3.26 4.1 4 3.71 3.75 P2O5 0.16 0.2 0.23 0.29 0.4 0.46 0.34 0 0.03 0 0.06 0 LOI 1.96 2 2 1.39 1.76 2.19 1.86 0.44 0.24 0.45 0.16 0.3 Total 101.1 100.36 99.53 98.94 100.61 100.26 99.96 100.09 98.62 100.27 99.18 100.31

Trace elements, ppm Y 1013171220181210914101

Nb41435441919201315 SCOTLAND FRONT, CALEDONIAN THE ALONG MAGMATISM EARLY Sr 1401 1163 1175 1279 1083 1095 1312 135 112 130 865 63 Rb 38 36 30 46 27 30 37 122 122 119 84 149 Zr 37 80 104 92 139 131 78 451 463 459 188 357 Ni5946181719211554654 Ba 1172 1179 1126 1618 946 1112 1622 87 267 146 924 157

Sample type Peralkaline Peralkaline Porphyritic Canisp Canisp rhyolite rhyolite trachyte porphyry porphyry Sample number BS15 BS23 GRO3 CP1 CP2

Major elements, wt.%

553 SiO2 76.03 76.28 66.26 67.28 68.8 TiO2 0.11 0.14 0.26 0.34 0.21 Al2O3 13.77 13.98 20.21 16.22 13.83 Fe2O3 tot 1.36 0.79 1.55 2.55 1.98 MnO 0.01 0 0 0.07 0.04 MgO 0 0 0 0.98 0.33 CaO 0.1 0 0 1.6 2.17 Na2O 5.66 5.7 8.33 5.79 4.1 K2O 3.24 3.67 2.76 3.43 3.46 P2O5 0.19 0.17 0 0.23 0.15 LOI 0.41 0.44 0.25 0.74 2.75 Total 100.88 101.17 99.62 99.23 97.82

Trace elements, ppm Y 1011151412 Nb 17 19 13 11 9 Sr 132 128 998 1489 199 Rb 116 121 50 82 78 Zr 431 451 15 125 177 Ni 5 6 14 13 10 Ba 149 497 1752 2135 3423 TABLE 3. New XRF data obtained by the BGS Moine Thrust project.

Sample type Porphyritic Peralkaline Porphyritic Porpyritic Nordmarkite Nordmarkite Nordmarkite Nordmarkite trachyte rhyolite Rhyolite trachyte trachyte Sample number KG 33 KG 34 KG 36 KG 46 KG10 KG40 KG43 KG67 KG71

Major elements, wt.% SiO2 63.73 56.62 59.92 64.61 64.78 75.79 72.69 65.46 62.58 TiO2 0.40 0.76 0.63 0.32 0.13 0.07 0.07 0.07 0.26 Al2O3 15.33 15.89 14.46 15.71 19.15 13.20 13.62 18.85 20.01 Fe2O3 tot 3.58 7.04 7.63 3.37 2.15 0.62 1.63 1.28 2.35 MnO 0.08 0.14 0.07 0.08 <0.01 <0.01 0.01 0.02 0.07 MgO 1.98 3.98 2.81 1.80 0.08 0.09 0.36 0.11 0.19 CaO 3.31 6.01 2.72 2.38 0.03 <0.01 0.49 0.17 0.90 Na2O 6.50 4.44 4.36 5.63 7.44 4.64 0.32 7.19 6.27 .M GOODENOUGH M. K. K2O 1.68 2.25 2.20 2.21 5.22 4.23 7.07 5.45 5.91 P2O5 0.19 0.33 0.31 0.15 0.01 <0.01 0.01 <0.01 0.02 LOI 2.46 1.81 4.18 2.87 0.62 0.53 3.18 0.70 0.73 Total 99.24 99.27 99.29 99.13 99.61 99.17 99.45 99.30 99.29 554 Trace elements, ppm

Rb 39 48 63 36 93 39 85 132 112 Ba 1210 864 782 1113 246 273 546 373 540 ETAL. Th 8 9 12 5 67 31 58 102 87 Nb7 81373132374737 La 26 37 46 25 59 10 91 28 118 Ce 57 74 87 50 109 27 179 58 183 Sr 958 1295 743 567 132 45 266 297 505 Nd 24 35 39 21 22 10 61 22 53 Zr 133 173 199 128 438 321 334 596 663 Sm2565331338 Y 11 16 18 8 17 13 34 6 23 Pb 12 16 13 12 109 14 266 121 113 Cu 5 43 15 5 13 10 11 13 26 Zn 41 79 71 49 43 10 57 19 68 Ni 19 32 20 23 <1 <1 <1 <1 <1 Sc7139 4<2<2<2<2<2 V 69 130 94 43 42 17 16 13 57 Cr 71 118 48 67 3 2 <2 <2 3 EARLY MAGMATISM ALONG THE CALEDONIAN FRONT, SCOTLAND

peralkaline rhyolites (Young, 1989). Although the existed for a significant period of time. It is clear mafic mineralogy of the peralkaline rhyolites is from field relationships that the vogesite, horn- now largely anhydrous, the internal morphology blende microdiorite and peralkaline rhyolite of the feldspar glomerocrysts, with plagioclase swarms were all emplaced prior to significant apparently havingundergone replacement by movements on the thrusts in the Moine Thrust alkali feldspar, and the presence of very coarse Belt. The Loch Ailsh Pluton must therefore also patch perthites, strongly indicates that a residual pre-date movements on these thrusts. (peralkaline) fluid phase was associated with the Alkaline magmatism continued in the area with rhyolitic magmas. We suggest that this served to the emplacement of the strongly alkaline, under- introduce certain incompatible elements, saturated Loch Borralan Pluton after movements includingNb and Zr, into the rhyolites. on the Ben More and, possibly, the Sole thrusts Since peralkaline rhyolite sills are seen cutting (see discussion in Parsons, 1999). The porphyritic the Loch Ailsh Pluton, which in turn hornfelses trachytes are alkali enriched in comparison with sills of vogesite, the evolving magma chamber members of the Hornblende Microdiorite Swarm from which the rhyolites were derived may have that have similar SiO2 contents (Figs 3 and 6).

FIG. 6. Harker diagrams for the Assynt minor intrusions, including whole-rock analyses from Tables 2 and 3 and Young(1989). Mineral analyses were obtained by electron microprobe studies of samples from the Assynt minor intrusions and were used by Young(1989) in his modellingcalculations.

555 K. M. GOODENOUGH ETAL.

This may indicate derivation from different magmas in Scotland to slab break-off following parental magmas, albeit possibly with the same the Scandian orogeny and the closure of the primary mantle source. Leucosyenites from the Iapetus ocean. Although we have shown that the Loch Borralan Pluton are similarly enriched in parental magmas for the majority of the Assynt alkalis (Thirlwall and Burnard, 1990) and plot minor intrusions formed prior to, or early during, close to the porphyritic trachytes on the total the main Scandian collisional event, we suggest alkali-silica diagram (Fig. 3). Since both the that the source of the Assynt magmas was Porphyritic Trachyte Swarm and the Loch influenced by the early stages of Scandian Borralan syenites appear to post-date movement subduction. on the Ben More Thrust, it is possible that the The question of why strongly alkaline magmas, porphyritic trachytes represent offshoots of the includinga carbonatite (Young et al., 1994), were later syenites of the Loch Borralan Pluton. emplaced into a region of active crustal short- However, as noted above, the porphyritic ening, remains an enigma. Ultrapotassic rocks trachytes have much lower La/Nb ratios than the associated with carbonatites are rare outside of syenites, so this hypothesis is uncertain. continental rift environments, but have also been The quartz-microsyenites of the Nordmarkite recorded in Italy, particularly at Mt. Vulture. The Swarm are chemically similar to the hornblende tectonic settingof the Mt. Vulture magmatism, microdiorites, but are moderately alkali enriched. close to the Apennine Front, is similar to that of As with the Canisp Porphyry sills, they could the NW Highlands intrusions. Recent work by theoretically have formed by fractionation of a Beccaluva et al. (2002) has suggested that the vogesite-type parental magma. However, it is source of these magmas may have been in important to bear in mind that substantial crustal lithospheric mantle that had undergone an shortening, perhaps as much as 100 km (Strachan earlier enrichment, possibly due to carbonatitic et al., 2002), has occurred between the intrusions metasomatism associated with extension, and was of the Nordmarkite Swarm east of the Moine then subsequently modified by subduction-related Thrust and the Canisp Porphyry sills of the metasomatism. A similar multi-stage process may Foreland. Thus, we prefer to consider the have generated the source of the alkaline magmas intrusions of the Nordmarkite Swarm as having of the NW Highlands. formed from a separate parental magma, although Further study into the ages and isotopic their chemical similarities to the hornblende systematics of the intrusions of the NW microdiorites probably indicate a similar Highlands is undoubtedly required in order to primary mantle source. fully investigate the sources of these magmas, and Most of the Assynt minor intrusions have Nb hence to understand the processes occurringalong troughs on their multi-element plots and this is the Caledonian Front duringthe Scandian generally considered to indicate the presence of orogeny (cf. Oliver, 2002). subduction-derived material in the mantle source of the magmas, although it is possible that crustal contamination could also produce a Nb trough. Conclusions The high Ba/Nb ratios of the majority of the We have used field, petrographical and geochem- Assynt magmas are typically only found in ical data to revise the classification of Sabine subduction-related settings (Fig. 7). The trace (1953) for the Assynt minor intrusions, part of the element ratios of the majority of the Assynt Northwest Highlands Minor Intrusion Suite. We minor intrusions, includingthe vogesites and recognize seven swarms of minor intrusions in the hornblende microdiorites, are similar to those of Assynt area. The vogesites, hornblende micro- many Caledonian igneous rocks across the diorites, and peralkaline rhyolites all occur within Northern Highlands. These include other lampro- the Assynt Culmination and were emplaced prior phyres (Canning et al., 1998); members of the to movement on the thrusts within the culmina- appinite suite (Fowler and Henney, 1996); and tion. The Canisp Porphyry occurs only within the major syenite plutons (Thompson and Fowler, Foreland and is also considered to pre-date 1986), and are largely geochemically similar to thrusting. The intrusions of the Ledmorite subduction-related shoshonitic suites (Thompson Swarm occur only within the Foreland and their and Fowler, 1986). relationships to thrust movements are unclear, Atherton and Ghani (2002) have related the although the strike of the dykes and their genesis of the majority of ‘Late Caledonian’ undersaturated character strongly suggest a

556 EARLY MAGMATISM ALONG THE CALEDONIAN FRONT, SCOTLAND

FIG. 7. Ba/Nb vs. Zr/Nb for the Assynt minor intrusions. Data from Tables 2 and 3 and from Young(1989). Data for other NW Highlands intrusions from Thompson and Fowler (1986) are shown for comparison, together with data from island arc volcanics from Merapi Volcano (Gertisser and Keller, 2003) and for average MORB and OIB (Sun and McDonough, 1989).

direct relationship with the Loch Borralan pluton, before the main compressional movements on the which was emplaced after movements on the Ben thrusts of the Assynt Culmination. The More Thrust had ceased. The porphyritic porphyritic trachytes are younger and more trachytes are found within the Assynt alkali-enriched than the other minor intrusions, Culmination, generally close to the Ben More and may be related to the Loch Borralan Pluton. Thrust, and post-date at least some movement on On the basis of the available chemical that thrust. The sills of the Nordmarkite Swarm evidence, it appears that the parental magmas are found close to the Moine Thrust and were for the Northwest Highlands Minor Intrusion emplaced prior to at least some ductile movement Suite were derived from a mantle source that alongthat thrust. was similar to that postulated for the major The mafic vogesites probably represent the intrusions of the NW Highlands, i.e. a mantle closest possible approximation to the parental source that has been modified by a subduction- magmas of the majority of the minor intrusion related component (Thompson and Fowler, 1986; suite. The hornblende microdiorites were formed Thirlwall and Burnard, 1990). Although the area by fractional crystallization of this parental over which these intrusions were emplaced may magma, and further fractionation generated have been >100 km in extent, it seems likely that rhyolitic magmas. These rhyolites were modified all the major and minor intrusions in Assynt were by a peralkaline, Nb and Zr-bearingresidual fluid ultimately derived from a similar mantle source. phase followingemplacement. The Canisp Some magmas generated from this source were Porphyry sills and the intrusions of the emplaced prior to the main movements on the Nordmarkite Swarm are chemically similar to Moine Thrust Zone. the hornblende microdiorites, but were spatially separated from them at the time of emplacement, Acknowledgements and brought into proximity by crustal shortening. Thus they were not generated in the same magma This paper has been written as part of the BGS chamber. Intrusion of all these swarms ceased Moine Thrust Project. Maarten Krabbendam is

557 K. M. GOODENOUGH ETAL.

thanked for invaluable discussions of the Assynt of Edinburgh: Earth Sciences, 71,69À96. minor intrusions and for comments on this Fowler, M.B. and Henney, P.J. (1996) Mixed manuscript. Much of the work from which this Caledonian appinite magmas: implications for paper is derived was carried out by one of the lamprophyre fractionation and high Ba-Sr granite authors (BY) as a Kilgour-funded PhD student- genesis. Contributions to Mineralogy and Petrology, ship at the University of Aberdeen. Mark Ingham, 126, 199À215. Charles Gowingand Neil Eatherington,BGS Gertisser, R. and Keller, J. (2003) Trace element and Keyworth, are thanked for preparation and Nd, Sr, Pb and O isotope variations in medium-K analysis of samples for XRF. Peter Sabine is and high-K volcanic rocks from Merapi Volcano, thanked for his advice and assistance, and Sally Central Java, Indonesia: Evidence for the involve- Gibson is thanked for her constructive review. ment of subducted sediments in Sunda Arc magma Sue Loughlin and David Stephenson provided genesis. Journal of Petrology, 44, 457À489. Gillespie, M.R. and Styles, M.T. (1999) BGS Rock much-appreciated comments on an earlier version Classification Scheme Volume 1. Classification of of this paper. KMG publishes with the permission Igneous Rocks. BritishGeological Survey Research of the Executive Director of the British nd report (2 edition), RR 99-06. Available at the BGS Geological Survey (NERC). website http://www.bgs.ac.uk/bgsrcs/ Halliday, A.N., Aftalion, M., Parsons, I., Dickin, A.P References and Johnson, M.R.W. (1987) Syn-orogenic alkaline magmatism and its relationship to the Moine Thrust Atherton, M.P. and Ghani, A.A. (2002) Slab breakoff: a Zone and the thermal state of the lithosphere in NW model for Caledonian, Late Granite syn-collisional Scotland. Journal of the Geological Society, 144, magmatism in the orthotectonic (metamorphic) zone 611À617. of Scotland and Donegal, Ireland. Lithos, 62,65À85. Heddle, M.F. (1881) The geognosy and mineralogy of Bailey, E.B. (1935) The Glencoul Nappe and the Assynt Scotland. À Continued. Mineralogical Culmination. Geological Magazine, 72, 151À165. Magazine, 4, 197À254. Beccaluva, L., Coltorti, M., Di Girolamo, P., Melluso, Horne, J. and Teall, J.J.H. (1892) On borolanite À an L., Milani, L., Morra, V. and Siena, F. (2002) igneous rock intrusive in the Cambrian limestone of Petrogenesis and evolution of Mt. Vulture alkaline Assynt, Sutherlandshire, and the sandstone volcanism (Southern Italy). Mineralogy and of Ross-shire. Transactions of the Royal Society of Petrology, 74, 277À297. Edinburgh, 37, 163À178. Bonney, T.G. (1883) Notes on a series of rocks from the Irvine, T.N. and Baragar, W.R.A. (1971) A guide to the North-West Highlands collected by C. Callaway, chemical classification of the common volcanic Esq. Quarterly Journal of the Geological Society, 39, rocks. Canadian Journal of EarthSciences , 8, 414À422. 523À548. Canning, J.C., Henney, P.J., Morrison, M.A., van Johnson, M.R.W. and Parsons, I. (1979) Geological Calsteren, P.W.C, Gaskarth, J.W. and Swarbrick, Excursion Guide to the Assynt District of Sutherland. A. (1998) The Fault: a major vertical Edinburgh Geological Society, Edinburgh. lithospheric boundary. Journal of the Geological Krabbendam, M. and Leslie, A.G. (2004) Lateral ramps Society, 155, 425À428. and thrust terminations: an example from the Moine Coward, M.P. (1983) The thrust and shear zones of the Thrust Zone, NW Scotland. Journalofthe Moine Thrust zone and the NW Scottish Geological Society, 161, in press. Caledonides. Journal of the Geological Society, Le Bas, M.J., Le Maitre, R.W., Streckeisen, A. and 140, 795À811. Zanettin, B. (1986) A chemical classification of Coward, M.P. (1985) The thrust structures of southern volcanic rocks based on the total alkali-silica Assynt, Moine thrust zone. Geological Magazine, diagram. Journal of Petrology, 27, 745À750. 122, 596À607. Le Maitre, R.W. and 11 others (1989) A Classification of Coward, M.P. and Potts, G.J. (1985) Fold nappes: Igneous Rocks and Glossary of Terms. examples from the Moine Thrust zone. Pp. Recommendations of the International Union of 1147À1158 in: The Caledonide Orogen À Geological Sciences Subcommission on the Scandinavia and Related Areas (D.G. Gee and Systematics of Igneous Rocks. Blackwell Scientific B.A. Sturt, editors). John Wiley & Sons Ltd., Publications, Oxford. Chichester, UK. McDonough, W.F. and Sun, S.-S. (1995) The composi- Elliott, D. and Johnson, M.R.W. (1980) Structural tion of the Earth. Chemical Geology, 120, 223À255. evolution in the northern part of the Moine thrust Oliver, G.J.H. (2002) Chronology and terrane assembly, belt, NW Scotland. Transactions of the Royal Society new and old controversies. Pp. 201À212 in: The

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